1. Field of the Invention
This invention relates to a solid pre-cast prestressed concrete foundation pile which is prestressed during manufacture by means of a single, central longitudinal prestressing strand (cable).
2. Description of Related Art
The effects of prestressing concrete members with steel strands has been long recognized as a means of providing additional bending strength necessary for the pre-casting of long slender units which require this added strength when being removed from the casting molds, when being handled in transport, and, in the case of foundation piles, preparatory to driving and during the driving process. It is recognized that concrete without reinforcement imparts very little resistance to tensile forces. Tensile forces occurring during driving must be accommodated and, although not a requirement in all pile applications, tension capacity in installed piles is frequently desired, in which case provision for connection to the other foundation structural elements must be made. Reinforcing and prestressing provide a means of developing the desired strength in bending and tension in pre-cast concrete piles.
Over time, several methods have been used to impart this additional strength through reinforcing. Historically, pre-cast prestressed piles have used a plurality of "prestressing strands" where each strand is composed of several high strength wires twisted together mechanically forming a single cable-like strand. These multiple strands are typically arranged in a uniform, symmetrical pattern near the edges of the pile's surface. This is usually combined with conventional reinforcing consisting of deformed steel rods and wire spiral, customarily for the full length of the pile or pile segment.
Several reinforcing methods have also been patented, however these relate to different configurations of reinforcing and/or to different pile types from the present invention which employs only one prestressing strand and employs wire spiral reinforcing in the ends only.
In U.S. Pat. No. 4,627,769 (Lee), a concrete foundation pile with a central removable threaded bar is demonstrated which requires anchorage plates and a steel capping plate, and incorporates a hollow sheath to permit the removal of the reinforcing bar for re-use with other piles.
U.S. Pat. No. 4,735,527 (Bullivant) relates to cylindrical pre-cast pile sections joined to form a continuous load-bearing pile with the sections approximately one meter in length and having at least one longitudinal rod with included spigot on one end and a socket on the other, the purpose of which is not only to reinforce the length of pile, but also to afford a means of mechanically attaching short segments to form a long complete pile.
U.S. Pat. No. 3,501,920 (Uchiyama) offers tubular concrete poles, piles or the like products having a plurality of non-tensioned ordinary-strength wires used in combination with conventional pretensioned high-strength steel wires all arranged at equal intervals in a common circle with auxiliary reinforcement wound spirally and from end to end around the axial reinforcement. The centrifugal compaction technique of fabrication is the suggested method of casting for this invention.
Common to all types of pre-cast concrete piles, whether pre-cast conventionally or prestressed, load-bearing foundation piles or concrete sheet piles as used in retaining walls, cushioning elements must be employed on the top of the piles when impact driving methods are used for the installation of the piles. Typically a block of wood or some other material is introduced between the pile top and the hammer accessory or base, usually by wedging or fixing the cushion material into the underside of the hammer or hammer accessory prior to placing the hammer on the pile. This method of cushioning has been employed in the United States for more than forty-five years and routinely presents a safety hazard to those installing the piles in that they must position themselves, or parts of their body, directly under the suspended piledriving hammer in order to install the cushion block in the hammer drive cap accessory or base.
In the case of U.S. Pat. No. 3,211,241 (Kikuchi) a metal protective or cap device is demonstrated, for use during driving, which covers the top of a rectangular pile as used in a retaining wall during driving and which device is bolted or fixed to the top of the pile and includes an open recess for the purpose of containing an impact receiving block.